Hall of Infinite Doors

You are submerged in quicksilver, and find yourself in a diamond cave. A silver serpent turns towards you.
"Hello, traveler. I have a question for you. If you answer correctly, you get to move ahead. If not, I will eat you."
You accept the offer.
"Alright, the question is :
WHAT IS THE GEM OF HAPPINESS IN THE CLASSIC SERIES "DELTORA QUEST"?"